Answer and Explanation: The Doppler effect is a change in the frequency of sound waves that occurs when the source of the sound waves is moving relative to a stationary listener. As the source of sound waves approaches a listener, the sound waves get closer together, increasing their frequency and the pitch of the sound.
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Answer:
The crests of the sound wave are spaced closer together.
Step-by-step explanation:
the wavelength between successive crests is decreased due to the motion of the truck in the same direction as the sound
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